At first, road from Vilnius to Kernave is about 36 km. Bus to Kernave is always drives from Vilnius bus station, so it is easy to find. Buses is always stop mostly in centre of Kernave - the best orientation of centre and most admirable places at this small town is a high tower of the main red church.
When you arrive in Kernave you'll see some carriages. If you want to have fun, try them.
How to reach it: I guess, the best way is to go there by car. From Kaunas-Vilnius highway close to Vievis turn to road No. 108 (right direction before Vievis from Vilnius side). Follow road directions to Kernave.
If you don't have that possibility, I'm sure you'll be able to have this trip from your hotel as a guided tour.